Electrochemical Properties Of Low Dimensional Vanadium Oxide Cathode Materials For Lithium-sulfur Battery

Lithium-sulfur batteries are one of the promising energy storage devices of the next generation due to the high energy density and high theoretical specific capacity of sulfur.However,the electronic insulating properties of sulfur and the product Li₂S,the bulk deformation effect of sulfur and the shuttle effect cause low sulfur utilisation of the active materials,short cycle life and low Coulombic efficiency,limiting the commercialisation of lithium-sulfur batteries.In this dissertation,vanadium oxides were prepared as the cathode for lithium-sulfur batteries to solve the polysulfides shuttle problem and improve the cycle stability of the batteries.V₂O₅ hollow microspheres,V₂O₅ nanosheets,VO₂ nanosheets and VO₂ nanorods were synthesised and mixed with highly conductive carbon materials to construct a variety of vanadium oxide/carbon material electrodes.Density Functional Theory（DFT）was performed on the binding energy of vanadium oxide and carbon materials to polysulfides.The microscopic composition of the vanadium oxide/carbon materials was investigated and the electrochemical impedance,cyclic voltammetry,cycling performance,rate performance,adsorption performance and performance enhancement mechanisms of the different electrode structures were systematically analysed.The main contents and results are as follows:（1）The hollow V₂O₅microspheres(V₂O_5MS)were synthesized by the solvothermal method and S@V₂O_5MS@GO composites with bilayer core-shell structure were formed by graphene oxide（GO）covered S@V₂O_5MS.The hollow structure of the V₂O₅microspheres bound the sulfur within the microspheres,relieving the volumetric deformation of the sulfur and binding the polysulfides.At the same time,V₂O_5MSprovided the active site for the adsorption of polysulfides.The oxygen-containing functional groups on the surface of graphene oxide anchored the polysulfides and the two-dimensional planar structure of the graphene oxide was further utilised to provide an ion-electron conducting network for the electrode.The capacity of S@V₂O_5MS@GO composites at 0.1 C reached 895.7 m Ah/g after 100 cycles at 0.1 C with a capacity retention of 69%and capacity retention of 97%after 200 cycles at 1 C.（2）To address the problem of capacity retention,V₂O₅ nanosheets(V₂O_5NS)and carbon nanotubes（CNTs）composites as the sulfur host materials were constructed.The binding energies of V₂O₅（001）crystalline surfaces to Li₂S₄,Li₂S₆ and Li₂S₈ were 7.09,5.53 and 4.67 e V,and the binding energies of carbon nanotubes to Li₂S₄,Li₂S₆ and Li₂S₈were 0.45,0.55 and 0.49 e V,respectively.Compared to carbon nanotubes,V₂O₅ had strong adsorption energy for polysulfides,and the high specific surface area and continuous conductive network of carbon nanotubes combined with the strong chemisorption of polar V₂O_5NS provided abundant active sites and shortened the electron transport pathway.The capacity retention of the S-V₂O_5NS-CNTs electrode was72.6%after 100 cycles at 0.1 C and 100%after 200 cycles at 1 C.（3）In situ self-assembled VO₂ nanosheets-reduced graphene oxide aerogel (VO_2NS-r GA)was prepared as sulfur host materials using a solvothermal method to address the issues of sulfur content（>70%）and long-term cycling stability.The polar VO_2NS could effectively anchor the polysulfides and accelerate the conversion of long-chain polysulfides to short-chain Li₂S₂/Li₂S,thus alleviating the polysulfide shuttle.In addition,the introduction of r GA increased the electrical conductivity of the sulfur electrode and effectively mitigated the volume expansion of sulfur during cycling,significantly enhancing the electrochemical reaction kinetics and demonstrating excellent cycling performance.The S-VO_2NS-r GA-c composites with a sulfur content of 75.4 wt%maintained a capacity of 861 m Ah/g after 100 cycles at 0.1 C with a capacity retention of 78.7%and a high capacity retention of 83%after 1000 cycles at 1C.（4）To address the problem of sulfur surface loading（≥2 mg/cm²）,three-dimensional interconnected interwoven VO₂ nanorods(VO_2NRs)-CNTs composites were prepared by a one-step solvothermal method as sulfur storage carriers for lithium-sulfur battery cathode.The binding energies of VO₂（110）crystalline surfaces to Li₂S₄,Li₂S₆ and Li₂S₈ were 2.90,4.04 and 2.62 e V,and the binding energies of carbon nanotubes to Li₂S₄,Li₂S₆ and Li₂S₈ were 0.45,0.55 and 0.49 e V,respectively.Compared with carbon nanotubes,VO₂ had a strong binding energy to polysulfides and accelerated the adsorption and catalytic conversion of polysulfides through the chemistry of polar surfaces,carbon nanotubes with high electrical conductivity can further accelerate the electrocatalytic process of Li PSs during charging and discharging,exhibiting outstanding electrochemical performance.The capacity retention of the S-VO_2NRs-CNTs electrode with a sulfur surface loading of 2 mg/cm² was 75.9%at 0.1 C for 100 cycles.（5）To further improve the large rate performance,VO_2NRs were prepared by the solvothermal method and mixed with graphene to construct a three-dimensional network structure.The VO_2NRs not only exhibited chemisorption of polysulfides but also catalysed polysulfides conversion.The binding energies of VO₂（110）crystalline surfaces to Li₂S₄,Li₂S₆ and Li₂S₈ were 2.90,4.04 and 2.62 e V,and the binding energies of graphene to Li₂S₄,Li₂S₆ and Li₂S₈ were 0.56,0.48 and 0.58 e V,respectively,the adsorption energy of VO₂ was greater than that of graphene.the combination of VO_2NRsand graphene achieved a synergistic effect of physical limitation and chemisorption/catalysis of polysulfides.In addition,the graphene folds can increase the material specific surface area and accelerate the electrochemical reaction kinetics.Electrochemical performance tests shown that the synthesised S-VO_2NRs-G electrode materials exhibited good performance.The S-VO_2NRs-G electrode with a sulfur content of 70.4 wt%exhibited a capacity retention of 89%after 100 cycles at 0.1 C and a capacity decay rate of 0.079%after 500 cycles at 2 C.